Exhaust gas treating apparatus

ABSTRACT

An exhaust gas processing device includes an air preheater for preheating air for combustion in a combustion device by using an exhaust gas emitted from the combustion device; a gas-gas heater heat recovery device composed of a heat transfer tube for recovering the heat of the exhaust gas to a heat medium; a dust collector; a wet-type desulfurization device; a gas-gas heater re-heater composed of a heat transfer tube for heating the exhaust gas at its outlet by using the heat medium supplied from the gas-gas heater heat recovery device, which are installed in that order from the upstream to the downstream of an exhaust gas duct of the combustion device.

TECHNICAL FIELD

The present invention relates to an exhaust gas processing device (alsoreferred to as a flue gas processing device) provided with a gas-gasheat exchanger suitable for exhaust gas re-heating, such as a boiler atthe outlet of a wet-type desulfurization device.

BACKGROUND ART

General exhaust gas processing systems are shown in FIG. 15, FIG. 10,and FIG. 11. In the exhaust gas processing system shown in FIG. 15, theexhaust gas containing a large amount of dust emitted from a combustiondevice such as a boiler 1 using coal as fuel is introduced to adenitration device 2 in which nitrogen oxide contained in the exhaustgas is removed. Then in an air preheater 3, the exhaust gas isheat-exchanged with air for combustion which is supplied to the boiler1. After most of the dust in the exhaust gas is removed in a dustcollector 5 (including a bag filter and an electric static precipitatorin the present specification), the exhaust gas is pressurized up by aninduced draft fan 6. Sequently, the exhaust gas is introduced to agas-gas heater (GGH) heat recovery device 4 in which heat is recovered,and then introduced to a wet-type desulfurization device 7 in whichsulfur oxide (SOx) contained in the exhaust gas is removed by gas-liquidcontacted with the absorber containing a desulfuring agent. The exhaustgas cooled down to the saturated gas temperature in the wet-typedesulfurization device 7 is pressurized up by the desulfuring fan 9,heated by a GGH re-heater 8, and emitted from a smokestack 10. Betweenthe GGH heat recovery device 4 and the GGH re-heater 8 are providedinterconnecting lines 13 in which a heat medium circulates.

The other exhaust gas processing systems are shown in FIG. 10 and FIG.11, and their GGH (gas-gas heater) systems are shown in FIGS. 12 and 13.In these drawings, the same components are referred to with the samereference numbers.

In FIG. 10, the exhaust gas from the boiler 1 is flown through anexhaust gas duct 30, introduced to the denitration device 2 in whichnitrogen oxide in the exhaust gas is removed, and in the air preheater3, is heat-exchanged with air for combustion to be supplied to theboiler 1. Next, the exhaust gas is introduced to the GGH heat recoverydevice 4 in which the exhaust gas is heat-exchanged with the heat mediumflowing through the heat recovery device 4, thereby decreasing thetemperature of the exhaust gas and also decreasing the electricresistance value of the dust in the exhaust gas. In this condition, theexhaust gas is introduced to an electric static precipitator 5 in whichmost of the dust in the exhaust gas is removed. Sequently, the exhaustgas is pressurized up by the induced draft fan 6, introduced to thewet-type exhaust gas desulfurization device 7, and subjected togas-liquid contact with a desulfuring agent-containing liquid so as toremove SOx and part of the dust in the exhaust gas. The exhaust gascooled down to the saturated gas temperature in the wet-typedesulfurization device 7 is heated by the GGH re-heater 8 by a heatexchange with the heat medium supplied from the heat recovery device 4,pressurized up by the desulfuring fan 9, and emitted from the smokestack10.

FIG. 11 shows a system where there is a wet-type dust collector 19 addedbetween the wet-type exhaust gas desulfurization device 7 and the GGHre-heater 8 in the exhaust gas duct 30 in order to further reduce thedust contained in the exhaust gas at the outlet of the wet-type exhaustgas desulfurization device 7.

In the exhaust gas processing systems shown in FIG. 10 and FIG. 11, theduct collector 5 is installed at a side down stream of the GGH heatrecovery device 4 in the exhaust gas duct 30, which results in adecrease in the temperature of the processing gas in the dust collector5, thereby decreasing the electric resistance of the dust and increasingthe efficiency of removing the dust. Thus, it has a high dust removingperformance, compared with the exhaust gas processing system shown inFIG. 15.

Since dust emission controls are becoming stricter recently, the exhaustgas processing systems shown in FIG. 10 and FIG. 11 have become mainstream processing systems for exhaust gas which contains a large amountof dust emitted from a boiler or the like using coal as fuel.

Next, the GGH systems of the exhaust gas processing systems shown inFIG. 10 and FIG. 11 will be described with reference to FIG. 12 and FIG.13.

The heat transfer tubes 11 in the GGH heat recovery device 4 and theheat transfer tubes 12-2 in the GGH re-heater 8 are connected with eachother via the interconnecting lines 13 where the heat medium iscirculated by a heat medium circulation pump 14. In the heat mediumcirculation system, there is a heat medium tank 15 installed forabsorbing the expansion of the heat medium in the system, and there isalso a heat medium heater 16 for controlling the temperature of the heatmedium so as to keep the operation of the boiler or the like stable. Thesteam drain generated in the heat medium heater 16 is recovered by aheat medium heater drain tank 17, and then transferred to a boiler-sidetank (not illustrated).

The heat transfer tubes 11 of the GGH heat recovery device and the heattransfer tubes 12-2 of the GGH re-heater are generally composed offin-equipped heat transfer tubes in order to improve the efficiency ofheat exchange. Furthermore, on the stage preceding the GGH re-heater 8is provided a bare tube 12-1 consisting of at least three stages of bareheat transfer tubes with no fins in order to remove (evaporate)corrosive mist scattering from the wet-type exhaust gas desulfurizationdevice 7.

Such a structure is disclosed in Japanese Published Unexamined PatentApplication No. 2000-161647 in which the heat medium circulating throughthe GGH heat recovery device 4 and the re-heater 8 is flown into thebare tube 12-1 so as to increase the surface temperature of the baretube, thereby removing the scattering mist.

FIG. 13 shows a system configuration where there is a SGH (steam gasheater) 20 installed as the heat transfer tubes composed of the baretube installed in the stage preceding the fin-equipped heat transfertubes 12-2 of the GGH re-heater 8 in the system shown in FIG. 12, andsteam is introduced to the SGH 20 from outside. The steam draingenerating in the SGH 20 is recovered by a SGH drain tank 18 and thentransferred to a boiler-side tank (not illustrated).

FIG. 14 shows a simplified side view (FIG. 14(a)) and a cross sectionalview taken along the line A-A (FIG. 14(b)) in the case where sootblowers 21 are installed as dust removers for the GGH.

The soot blowers 21 used in the GGH are generally kept inside theexhaust gas duct 30 because the exhaust gas temperature in the GGH islow (160° C. or lower). When the soot blowers 21, which are suppliedwith steam or air, are in operation, the tubes inserted in the sootblowers 21 go back and forth while rotating (moving vertically in thecase shown in FIG. 14), and during movement, steam or air is jetted fromthe holes formed in the tubes, thereby removing dust and the likeaccumulated in the heat transfer tubes (fin-equipped heat transfertubes) 11 and 12-2 of the GGH.

In general, in a heat exchanger with GGH heat transfer tubes(fin-equipped heat transfer tubes), the heat transfer performance of theheat exchanger can be improved by increasing the flow rate of the gaswhich passes through the heat transfer tube region, thereby reducing thetotal heat transfer area.

Diminishing the fin pitch of the fin-equipped heat transfer tubes usedas the heat transfer tubes (in general, the fin pitch is not more than5.08 mm) can increase the heat transfer area per heat transfer tube, soas to reduce the number of heat transfer tubes installed in the wholeheat exchanger, thereby reducing the size of the heat exchanger.

However, in the aforementioned exhaust gas processing system providedwith the GGH, the exhaust gas introduced to the GGH heat recovery device4 installed at a side down stream of the air pre-heater 3 (a side upperstream of the dust collector 5) in the exhaust gas duct 30 contains alarge amount of dust (10 to 50 g/m³N or so). This causes a problem ofabrasion (due to ash erosion) over time with the heat transfer tubes 11of the GGH heat recovery device 4 and their fins, and also the problemof clogging of the regions between the adjacent fins as a result thatthe dust and SO₃ contained in the exhaust gas adhere to the heattransfer tubes 11.

In the GGH re-heater 8 installed at a side down stream of the wet-typeexhaust gas desulfurization device 7 in the exhaust gas duct 30, thedust collector 5 and the wet-type exhaust gas desulfurization device 7remove dust, so its amount is reduced to approximately 20 mg/m³N orlower. Consequently, in the GGH re-heater 8, the abrasion (ash erosion)environment due to the dust is mitigated. However, there are still otherproblems as follows. The sulfur oxide absorber containing plaster slurryand the like and mist containing a corrosive ingredient which scattersfrom the devices (the wet-type exhaust gas desulfurization device 7 andthe wet-type dust collector 19) installed at a side down stream of theGGH re-heater 8 collide with the fin-equipped heat transfer tubes 12-2of the GGH re-heater 8, thereby corroding the fin-equipped heat transfertube 12-2. In addition, the dust adhered to the fins over time clogs theregions between the adjacent fins and between the adjacent heat transfertubes where the gas flows.

In general, the soot blowers 21 or the like are installed as GGH dustremovers, and for effective dust removal of the heat transfer tubescomposing the GGH, it is necessary to take some measures, such asincreasing the number of soot blowers 21 or increasing the frequency ofactivating the soot blowers 21.

Normally, the soot blowers 21 are activated (timer control) at afrequency of 3 to 5 times a day. Since the operation of the soot blowers21 is controlled at a frequency of activation based on the worstconditions assumed in consideration of the problems that the dustadheres to the fins of the fin-equipped heat transfer tubes and clogsthe regions between the adjacent fins and between the adjacent heattransfer tubes where the gas flows, an excessive amount of steam tendsto be introduced to the duct 30.

Therefore, the object of the present invention is to provide an exhaustgas processing device provided with heat transfer tubes for the GGH heatrecovery device and the GGH re-heater, which are structured to solve theaforementioned problems in consideration of the environment with a largeamount of dust where the GGH is installed.

DISCLOSURE OF THE INVENTION

The aforementioned object of the present invention is solved by thefollowing constitutions. Namely an exhaust gas processing devicecomprising: an air preheater for preheating air for combustion in acombustion device by using an exhaust gas emitted from the combustiondevice; a gas-gas heater heat recovery device composed of a heattransfer tube for recovering the heat of the exhaust gas at the outletof the air preheater to a heat medium; a dust collector for collectingdust in the exhaust gas at the outlet of the gas-gas heater heatrecovery device; a wet-type desulfurization device for removing sulfuroxide in the exhaust gas at the outlet of the dust collector; a gas-gasheater re-heater composed of a heat transfer tube for heating theexhaust gas at the outlet of the wet-type desulfurization device byusing the heat medium supplied from the gas-gas heater heat recoverydevice, which are arranged in that order from the upstream side to thedownstream side of an exhaust gas duct of the combustion devices; and aheat medium circulation line for connecting heat transfer tubes providedin each of the gas-gas heater heat recovery device and the gas-gasheater re-heater and for circulating the heat medium through the heattransfer tubes, wherein the heat transfer tube of the gas-gas heaterheat recovery device is squarely arranged in the gas flow direction insuch a manner that the inter-tube flow rate, which is the flow rate ofthe exhaust gas between the heat transfer tubes adjacent in thedirection orthogonal to the gas flow direction, can be 10 m/s or lower.

The exhaust gas processing device is also able to structured in such amanner that the dust collector of the wet type is disposed in theexhaust gas duct between the wet-type desulfurization device and thegas-gas heater re-heater.

It is preferable that at least some of the heat transfer tubes of thegas-gas heater are composed of fin-equipped heat transfer tubes; the finpitch of the heat transfer tubes of the gas-gas heater heat recoverydevice is set to 7.25 to 10.16 mm; and the fin pitch of the heattransfer tubes of the gas-gas heater re-heater is set to 6.35 to 8.47mm, and it is also preferable that at least three stages of heattransfer tubes composed of a bare tube are installed on the stagepreceding the fin-equipped heat transfer tubes of the gas-gas heaterre-heater; and that the bare tube is a staggered arrangement in the gasflow direction so that the inter-tube flow rate, which is the flow rateof the exhaust gas between the heat transfer tubes adjacent in thedirection orthogonal to the gas flow direction, cannot be more than 12to 16 m/s.

The heat transfer tubes composed of the bare tube installed in the stagepreceding the fin-equipped heat transfer tubes of the gas-gas heaterre-heater can be either made a part of the heat medium circulation linefor circulating the heat medium through the gas-gas heater heat recoverydevice and the gas-gas heater re-heater, or made a steam line forflowing steam that is installed separately from the heat mediumcirculation line.

It is also preferable that the heat transfer tubes of the gas-gas heaterheat recovery device are tied in bundles each having a prescribed numberof heat transfer tubes; the bundles are each composed of heat transfertubes of not more than eight stages arranged in the gas flow directionand have a width of 3000 mm or less in the direction orthogonal to thegas flow direction, and in front and in back of the bundles in the gasflow direction are installed dust removers.

The exhaust gas processing device can be also structured in such amanner that either the gas-gas heater heat recovery device or thegas-gas heater re-heater is provided with dust removers; differentialpressure gauges and/or thermometers are provided in front and in back ofthe bundles in the gas flow direction; and control devices are providedto initiate the dust removers when the measured values of thedifferential pressure gauges and/or the thermometers become prescribedvalues or higher or lower.

The problems on the GGH heat recovery device 4 side, which are theabrasion of the fin-equipped heat transfer tubes 11 due to ash andclogging of the fin-equipped heat transfer tube parts 11, can be solvedby defining the specification of the GGH heat recovery device 4 asfollows.

In the exhaust gas processing system shown in FIG. 15, it is unnecessaryto take measures against the abrasion of the GGH heat transfer tubesbecause the GGH heat recovery device 4 is installed at a side downstream of the dust collector 5 in the exhaust gas duct 30, and a heatexchange is performed in the GGH heat recovery device 4 by using theexhaust gas after most of the dust has been captured by the dustcollector 5. However, as mentioned earlier, in recent years, the exhaustgas processing systems shown in FIG. 10 and FIG. 11 have becomemainstream processing systems because of their higher dust removingefficiencies than the exhaust gas processing system shown in FIG. 15.

However, as shown in FIG. 10 and FIG. 11, since the GGH heat recoverydevice 4 is installed at a side upper stream of the dust collector 5,the exhaust gas introduced to the GGH heat recovery device 4 contains alarge amount of dust (10 to 50 g/m³N or so), with concern that the heattransfer tubes may be seriously worn. In general, the amount of abrasionof the heat transfer tubes is affected by the gas flow rate, the dustconcentration, and the like in the exhaust gas. Although it depends onthe specification of the fin-equipped heat transfer tubes, in order touse fin-equipped heat transfer tubes as a GGH, it is generallypreferable that the abrasion rate in practice is approximately 0.1mm/year or less as indicated by the abrasion limit line A shown in FIG.6.

As a result of studies regarding methods for preventing the abrasion ofthe heat transfer tubes of the GGH heat recovery device in the casewhere the GGH heat recovery device is installed at a side upper streamof the dust collector, the inventors of the present invention have founda relationship between the abrasion amount of the GGH heat transfertubes, the gas flow rate, and the dust concentration in the gas.

To be more specific, as shown in FIG. 6, it has been confirmed that theinstallment of the GGH heat recovery device at a side upper stream ofthe dust collector causes an increase in the amount of abrasion of theheat transfer tubes with increasing gas flow rate. However, to oursurprise, it has been found that at a gas flow rate of 10 to 11 m/s ormore, the amount of abrasion rapidly increases, regardless of the levelof dust concentration in the exhaust gas.

Therefore, as a precaution against ash erosion due to the dust containedin the gas, the inter-tube flow rate of the gas which passes through theheat transfer tubes (fin-equipped heat transfer tubes) of the GGH heatrecovery device is regulated at 10 m/s or lower, which makes it possibleto prevent abrasion of the heat transfer tubes of the GGH heat recoverydevice, while keeping the dust removing efficiency high.

The term the inter-tube flow rate in the fin-equipped heat transfertubes indicates, in the horizontal cross sectional view of the heattransfer tubes shown in FIG. 1(a), the flow rate of the gas passingthrough the space cross sectional region, which is obtained bysubtracting the total cross sectional areas of the heat transfer tubes11 a, 12-2 a of the fin-equipped heat transfer tubes 11 and 12-2 shownin FIG. 12 and FIG. 13 and the fins 11 b and 12-2 b orthogonallycrossing the direction G of the gas flow from the cross sectional areaof the duct orthogonally crossing the direction G of the gas flow on thecenter axis L of the heat transfer tubes on the first stage in the gasflow direction.

FIG. 7 shows the comparative data of the pressure loss ratios of gasflow in the case where fin-equipped heat transfer tubes which differ infin pitch are arranged for three days in processing gas flows whichdiffer in dust concentration, including zero dust concentration. In thecase where the heat transfer tubes are bare tubes without fins, thepressure losses are nearly fixed when the gases pass through the heattransfer tubes, regardless of the dust concentration. On the other hand,in the case of the fin-equipped heat transfer tubes, dust tends to clogthe regions between the adjacent fins as the fin pitch decreases, whichgreatly contributes to an increase in the pressure loss of the gas flowand a decrease in the heat transfer performance of the heat transfertubes.

In general, in the heat exchanger provided with the fin-equipped heattransfer tubes, the heat transfer area per heat transfer tube increaseswith decreasing fin pitch. Therefore, the total number of heat transfertubes installed in the GGH for provision of the heat transfer areanecessary for the heat exchange can be reduced. Consequently, when theprocessing gas is a clean gas (contains no dust at all), the fin pitchis generally set in the range of 5.08 mm or lower from an economicstandpoint.

However, since the processing gas contains dust and the like, it isnecessary to set the fin pitch of the fin-equipped heat transfer tubes11 and 12-2 of the GGH heat recovery device 4 and the GGH re-heater 8 atan appropriate value. As shown in FIG. 7, in an environment involvingprocessing gas which contains dust and the like, the optimum applicationrange of the fin pitch is determined by taking into consideration thatthe cross sectional area of the exhaust gas duct decreases over time dueto adhesion of dust on the heat transfer tubes, and consequently thepressure loss of the gas flow while the gas passes through the heattransfer tubes increases over time.

To be more specific, for the purpose of preventing clogging of thefin-equipped heat transfer tubes due to adhesion of soot dust and theSO₃ and the like contained in the exhaust gas, and improving the dustremoving effects of the dust removers such as the soot blowers 21, thefin-equipped heat transfer tubes of the GGH heat recovery device 4 aresquarely arranged, and the fin pitch of the heat transfer tubes of theGGH heat recovery device 4 is set to 7.25 to 10.16 mm.

The term, fine pitch indicates the interval F of the adjacentindependent fins of the heat transfer tubes shown in FIG. 1(b) which isa partial side view of the heat transfer tubes, or the pitch F of thefins wound around the heat transfer tubes shown in FIG. 1(c).

FIG. 8 is a view showing changes over time in pressure loss (AP) of theexhaust gas flow in the GGH heat transfer tube region. The pressure lossin the heat transfer tube region tends to increase gradually over time.As shown in FIG. 8(a), when the number of stages of the heat transfertubes in the gas flow direction is eight or lower, the pressure loss canbe recovered nearly to the initial value by operating the soot blowersat the timing indicated with S/B in FIG. 8. On the other hand, whenthere are more than eight stages of heat transfer tubes as shown in FIG.8(b), the initial pressure loss cannot be recovered when the sootblowers are operated. In general, a GGH is composed of a necessarynumber of bundles of heat transfer tubes for heat exchange, and the sametendency is seen in the case where the bundles of the heat transfertubes are installed in a width not less than 3000 mm (the width in thedirection orthogonal to the gas flow).

Consequently, in the structure of the heat transfer tubes of the GGHheat recovery device 4, for the purpose of improving the dust removingperformance of the soot blowers 21 which are dust removers, the size ofthe bundles is defined in such a manner as to have eight or less stagesin the gas flow direction and to be 3000 mm or less in the widthdirection, and the soot blowers 21 are installed in front and in back ofthe bundles in the gas flow direction.

On the other hand, the dust concentration in the exhaust gas flown intothe GGH re-heater 8 installed at a side down stream of the wet-typeexhaust gas desulfurization device 7 is as small as 20 mg/m³N or lower,which makes it unnecessary to take ash erosion into consideration, andthe gas flow rate is limitless. As a result, an appropriate gas flowrate can be selected from the relationship with the pressure loss of theexhaust gas passing through the heat transfer tube region of the GGHre-heater 8. However, in the GGH re-heater 8, the absorber-containingmist scattering from the devices (the desulfurization device 7 and thewet-type dust collector 19) installed at a side down stream of the GGHre-heater 8 in the duct 30 collide with the fin-equipped heat transfertubes 12-2, with concern that the fin-equipped heat transfer tube 12-2may be corroded.

FIG. 9 shows the relationship between the inter-tube gas flow rate, thepressure loss, and the mist removing performance of the bare tubes 12-1of the GGH re-heater 8.

The mist removing performance and the pressure loss of the gas flow inthe heat transfer tube region depend on the gas flow rate, and with anincrease in the gas flow rate, both the mist removing rate (solid linea) and the aforementioned pressure loss ratio (broken line b) increase.

It is necessary that the mist removing rate is not less than 60%, and ithas been confirmed that the mist removing rate has nearly a fixed valuewhen the gas flow rate is 16 m/s or over. This is because the mistscattering from the wet-type exhaust gas desulfurization device 7 isremoved by colliding with the bare tube due to inertial impaction;however, mist particles with a diameter smaller than a certain valuefollow the exhaust gas flow, without colliding with the bare tube.

In general, the amount of mist at the inlet of the GGH re-heater 8 (theoutlet of the wet-type exhaust gas desulfurization device 7) is 100 to150 mg/m³N or so. In order to mitigate the corrosion environment in thefin-equipped heat transfer tubes 12-2 of the GGH re-heater 8 and toperform a stable operation in such conditions, it is generallypreferable that the mist removing efficiency rate is 60% or higher. As aresult, it has been confirmed that the most effective application rangeof the bare tubes 12-1 of the GGH re-heater 8 is in an inter-tube gasflow rate of 12 to 16 m/s.

Therefore, at least three stages of heat transfer tubes (bare tubes)12-1 are a staggered arrangement in the gas flow direction on the stagepreceding the fin-equipped heat transfer tubes 12-2 of the GGH re-heater8, and the inter-tube flow rate of the bare tubes 12-1 is defined in therange of 12 to 16 m/s.

By installing the bare tubes 12-1 and setting the fin pitch of thefin-equipped heat transfer tubes 12-2 of the GGH re-heater 8 to 6.35 to8.47 mm, clogging of dust in the fin-equipped heat transfer tubes 12-2over time and other problems can be solved, thereby realizing a morestable operation.

The inter-tube flow rate of the bare tubes 12-1 indicates, in thearrangement of the heat transfer tubes shown in the plan view of FIG. 2,the flow rate of the gas passing through the projected cross sectionalarea, which is obtained by subtracting the projected cross sectionalarea of the bare tubes 12-1 from the cross sectional area of the duct onthe center axis L of the bare tubes 12-1 on the first stage in the gasflow direction.

In the case where soot blowers are installed as the dust removers of theGGH heat recovery device or the GGH re-heater, it is possible to operatethe soot blowers when necessary by providing means for measuringdifferential pressure values or temperatures in front and in back ofeach of the heat transfer tube bundles of the GGH heat recovery deviceor the GGH re-heater in the gas flow direction, and by activating thesoot blowers when the differential pressure values or the temperaturesbecome prescribed values or higher (or lower), thereby preventing thesoot blowers from using an excessive amount of steam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the specification of the GGH heat transfer tubes of theembodiment of the present invention.

FIG. 2 shows the specification of the bare tube at a side upper streamof the fin-equipped tubes of the GGH re-heater of the embodiment of thepresent invention.

FIG. 3 shows block diagrams of the GGH heat transfer tubes of theembodiment of the present invention.

FIG. 4 shows structural diagrams where there are means for measuringdifferential pressure values or temperatures provided in front and inback of each of the GGH bundles of the embodiment of the presentinvention.

FIG. 5 shows structural diagrams where there are means for measuringdifferential pressure values or temperatures provided in front and inback of each of the GGH bundles of the embodiment of the presentinvention.

FIG. 6 shows the relationship between the amount of abrasion, the gasflow rate, and the dust concentration in the heat transfer tubes of theembodiment of the present invention.

FIG. 7 shows the relationship between the fin pitch and the pressureloss of the embodiment of the present invention.

FIG. 8 shows changes over time in pressure loss of the heat transfertube part of the embodiment of the present invention.

FIG. 9 shows the relationship between the gas flow rate, the pressureloss, and the mist removing performance in the bare tube part of theembodiment of the present invention.

FIG. 10 shows a general exhaust gas processing system.

FIG. 11 shows a general exhaust gas processing system in the case wherea SGH is installed.

FIG. 12 shows a general system in the vicinity of the GGH.

FIG. 13 shows a general system in the vicinity of the GGH in the casewhere a SGH is installed.

FIG. 14 shows a simplified side view when soot blowers are installed asdust removers (FIG. 14(a)) and a cross sectional view taken along theline A-A of FIG. 14(a) (FIG. 14(b)).

FIG. 15 shows a conventional general exhaust gas processing system.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiment of the present invention will be described with referenceto the drawings as follows. The embodiment of the present inventionutilizes the exhaust gas processing systems shown in FIG. 10 and FIG. 11and the GGH (gas-gas heater) systems shown in FIG. 12 and FIG. 13.

FIG. 1 shows the specification of the heat transfer tubes (fin-equippedheat transfer tubes) 11 and 12-2 respectively used in the GGH heatrecovery device 4 and the re-heater 8 of the present embodiment. Asshown in the horizontal cross sectional view of the heat transfer tubesof FIG. 1(a), the heat transfer tubes (fin-equipped heat transfer tubes)11 of the GGH heat recovery device 4 are squarely arranged in the gasflow direction, and the fin pitch of the fin-equipped heat transfertubes 11 is set at 7.25 to 10.16 mm. Furthermore, the heat transfertubes 11 are arranged in such a manner that the gas passes through thefin-equipped heat transfer tubes 11 of the GGH heat recovery device 4 atan inter-tube flow rate of 10 m/s or lower.

The fin pitch of the fin-equipped heat transfer tubes 12-2 of the GGHre-heater 8 is set to 6.35 to 8.47 mm.

Table 1 shows typical values of the inter-tube flow rate, diameter, andpitch of the heat transfer tubes, and the diameter and pitch of the finsin the fin-equipped heat transfer tubes 11 and 12-2. TABLE 1 heatrecovery device re-heater inter-tube flow rate V (m/s) ≦10 — heattransfer tube diameter 30 to 40 30 to 40 d (mm) fin diameter DF (mm) 60to 80 60 to 80 heat transfer pitch P (mm) 90 to 120 90 to 120 fin pitch7.25 to 10.16 6.35 to 8.47

FIG. 2 shows the horizontal cross sectional view of the heat transfertubes (bare tubes) 12-1 to be installed in the stage preceding the GGHre-heater 8 according to the present invention. As shown in thehorizontal cross sectional view of FIG. 2, on the stage preceding theGGH re-heater 8 are provided at least three stages of heat transfertubes (bare tubes) 12-1 in a staggered arrangement in the gas flowdirection. By setting the inter-tube flow rate of the gas passingthrough the heat transfer tubes (bare tubes) 12-1 in the range of 12 to16 m/s, 60% or more of the mist scattering from the devices on the upperside can be removed effectively, thereby mitigating the corrosiveenvironment of the fin-equipped heat transfer tubes 12-2 of the GGHre-heater 8. In this case, the heat transfer tubes (bare tubes) 12-1 arein a staggered arrangement so as to make P (heat transfer tube pitch)/d(heat transfer tube diameter)<2 for the purpose of improving theevaporating efficiency of the mist.

Table 2 shows typical values of the inter-tube flow rate, heat transfertube diameter, heat transfer tube pitch, and number of stages of theheat transfer tubes (bare tubes) 12-1 used in the GGH re-heater 8. TABLE2 a bare tube inter-tube flow rate V (m/s) 12 to 16 heat transfer tubediameter d (mm) 30 to 40 heat transfer tube pitch P (mmn) 75 to 90number of stages of heat transfer tubes ≧3 (stages)

The inter-tube flow rate of the bare tubes 12-1 indicates the flow rateof the gas which passes through the space part corresponding to the areaobtained by subtracting the projected cross sectional area of the baretubes 12-1 in the gas flow direction from the cross sectional view ofthe duct on the center axis L of the heat transfer tubes of the firststage in the gas flow direction in the heat transfer tube arrangementshown in FIG. 2.

Next, the simplified view of the bundles of the GGH heat transfer tubesof the present embodiment will be shown.

FIG. 3 is an example of bundles formed into blocks by combining the heattransfer tubes, as the constitutional unit of the heat transfer tubes(fin-equipped heat transfer tubes) 11 and 12-2 which flow the heatmedium M used in the GGH heat recovery device 4 and the re-heater 8.FIG. 3 shows simplified views of an upright structure (pendant type) ofthe heat transfer tube bundles of the GGH: FIG. 3(a) shows a side viewof the heat transfer tube bundles in the direction of the exhaust gasflow; FIG. 3(b) is a partly enlarged view of FIG. 3(a); and FIG. 3(c) isa side view of the heat transfer tube bundles in the directionorthogonal to the exhaust gas flow.

In order to improve the dust removing effects of the soot blowers 21,each bundle shown in FIG. 3 is structured in such a manner as to haveeight or less stages in the gas flow direction and to have a bundlewidth (the width in the direction orthogonal to the gas flow) of notmore than 3000 mm, and the soot blowers 21 are installed in front and inback of each bundle in the direction of the exhaust gas flow.

As shown in FIG. 3, each heat transfer tube bundle is suspended fromheaders 23 placed on the bundle support beams 25 provided overreinforcing columns 24. The headers 23 are connected with nozzles 22 forheat transfer tube headers.

Thus forming the GGH heat transfer tube bundles into an uprightstructure (pendant type) decreases the number of inner supports to beinstalled in the GGH, thereby reducing the total weight of the GGH.Moreover, the installing operation of the GGH becomes easy, making theon-site installation work comparatively easy.

FIG. 4(a), FIG. 4(b), FIG. 5(a), and FIG. 5(b) show structures which arerespectively provided with differential pressure gauges 27 andthermometers 28 installed in front and in back of each heat transfertube bundle of the GGH of the present embodiment in the gas flowdirection, and also provided with control devices 31 for controlling thedriving of the motors 32 of the soot blowers 21 by signals from thesemeasuring means 27 and 28.

The soot blowers 21, which are the dust removers of the heat transfertubes, are activated by timer control or the like at a frequency of 3 to5 times a day. With such control, the soot blowers 21 are operated at afrequency of activation based on the worst conditions assumed so as toprevent the dust adhered to the heat transfer tubes from clogging in theregions between the adjacent heat transfer tubes. This tends to causethe introduction of an excessive amount of steam.

In view of this problem, in the embodiment shown in FIG. 4, the sootblowers 21 are activated when the differential pressure values of thedifferential pressure gauges 27 or the temperatures of the thermometers28 reach prescribed values or higher (or lower).

INDUSTRIAL APPLICABILITY

As described hereinbefore, in the structure of an exhaust gas processingsystem provided with a gas-gas heat exchanger (GGH) as means forremoving soot dust and sulfur oxide contained in the exhaust gas emittedfrom a boiler or the like and for re-heating the exhaust gas emittedfrom the exhaust flue, the application of the specification andstructure of the GGH heat transfer tubes according to the presentinvention makes it possible to reduce abrasion and corrosion of the finsover time, and also clogging of the regions between the adjacent finsand between the adjacent heat transfer tubes due to the adhesion of thedust, SO₃, and absorber, which realizes a stable operation in the devicefor processing the exhaust gas emitted from the boiler and the like.

Furthermore, as a method for operating the soot blowers as dust removersof the GGH, it is possible that the amount of utility used can beminimized by activating the soot blowers when the differential pressurevalues and the temperatures in front and in back of each of the GGHre-heater bundles in the gas flow direction reach prescribed values orhigher (or lower).

1. An exhaust gas processing device comprising: an air preheater forpreheating air for combustion in a combustion device by using an exhaustgas emitted from the combustion device; a gas-gas heater heat recoverydevice composed of a heat transfer tube for recovering the heat of theexhaust gas at the outlet of the air preheater to a heat medium; a dustcollector for collecting dust in the exhaust gas at the outlet of thegas-gas heater heat recovery device; a wet-type desulfurization devicefor removing sulfur oxide in the exhaust gas at the outlet of the dustcollector; a gas-gas heater re-heater composed of a heat transfer tubefor heating the exhaust gas at the outlet of the wet-typedesulfurization device by using the heat medium supplied from saidgas-gas heater heat recovery device, being arranged in that order fromthe upstream side to the downstream side of an exhaust gas duct of thecombustion device; and a heat medium circulation line for connectingheat transfer tubes provided in each of the gas-gas heater heat recoverydevice and the gas-gas heater re-heater and for circulating the heatmedium through the heat transfer tubes, wherein the heat transfer tubeof the gas-gas heater heat recovery device is squarely arranged in thegas flow direction in such a manner that the inter-tube flow rate, whichis the flow rate of the exhaust gas between the heat transfer tubesadjacent in the direction orthogonal to the gas flow direction, can be10 m/s or lower.
 2. The exhaust gas processing device according to claim1, wherein the dust collector of the wet type is disposed between thewet-type desulfurization device and the gas-gas heater re-heater in theexhaust gas duct.
 3. The exhaust gas processing device according toclaim 1, wherein at least some of the heat transfer tubes of the gas-gasheater are composed of fin-equipped heat transfer tubes, and the finpitch of the heat transfer tubes of the gas-gas heater heat recoverydevice is set at 7.25 to 10.16 mm, and the fin pitch of the heattransfer tubes of the gas-gas heater re-heater is set at 6.35 to 8.47mm.
 4. The exhaust gas processing device according to claim 1, whereinat least three stages of the heat transfer tubes composed of a bare tubeare installed on the stage preceding the fin-equipped heat transfertubes of the gas-gas heater re-heater, and said bare tube is staggeredarrangement in the gas flow direction so that the inter-tube flow rate,which is the flow rate of the exhaust gas between the heat transfertubes adjacent in the direction orthogonal to the gas flow direction,cannot be more than 12 to 16 m/s.
 5. The exhaust gas processing deviceaccording to claim 4, wherein the heat transfer tubes composed of thebare tube installed in the stage preceding the fin-equipped heattransfer tubes of the gas-gas heater re-heater are either made a part ofthe heat medium circulation line for circulating the heat medium throughthe gas-gas heater heat recovery device and the gas-gas heaterre-heater, or made a steam line for flowing steam that is installedseparately from said heat medium circulation line.
 6. The exhaust gasprocessing device according to claim 1, wherein the heat transfer tubesof the gas-gas heater heat recovery device are tied in bundles eachhaving a prescribed number of heat transfer tubes; the bundles are eachcomposed of heat transfer tubes of not more than eight stages arrangedin the gas flow direction and have a width of 3000 mm or less in thedirection orthogonal to the gas flow direction, and in front and in backof the bundles in the gas flow direction are installed dust removers. 7.The exhaust gas processing device according to claim 5, wherein eitherthe gas-gas heater heat recovery device or the gas-gas heater re-heateris provided with dust removers; differential pressure gauges and/orthermometers are provided in front and in back of the bundles in the gasflow direction; and control devices are provided to activate the dustremovers when the measured values of the differential pressure gaugesand/or the thermometers become prescribed values or higher or lower.